Field of the Invention
The present invention relates to a projection optical system such as one suitable as a projection optical system for use in an image projection apparatus (projector) enlarging and projecting an image on a screen.
Description of the Related Art
Conventionally, various image projection apparatuses (projectors) using an image display element such as a liquid crystal display element and enlarging and projecting an image consisting of the image display element on a screen with use of a projection optical system have been discussed. The projection optical system for use in the projector is desired to be a zoom lens enabling projection at various magnifications and having a wide viewing angle and high optical performance sufficient to enable the image to be projected on a large screen at close range, as well as a projection optical system having a fixed focal length. To arrange a color separation optical system and a color combining optical system between the projection optical system and the image display element, the projection optical system is also desired to have a long back focus and have favorable telecentricity.
Conventionally, zoom lenses for the projector having a wide viewing angle, a long back focus, and favorable telecentricity are known (US2011/0109976, US2009/0195884, US2005/0200967). Each of the zoom lenses discussed in these conventional techniques includes a first to sixth lens units respectively having negative, positive, positive, negative, positive, and positive refractive power in this order from an enlargement conjugate side to a reduction conjugate side. These conventional techniques also discuss a zoom lens having a wide viewing angle in which the first and sixth lens units do not move while the second to fifth lens units move at the time of zooming.